Low voltage, regulated direct current power supplies are commonly incorporated in many products. Personal computers, automobiles, and home appliances are examples of devices that require a regulated, direct current voltage derived from an alternating current energy source.
Home appliances that plug into a wall socket receive their energy from the electric utility providing a highly regulated 60 cycle (50 cycle in Europe) alternating current signal having a voltage that falls within a defined range. This alternating current signal must be stepped down in voltage and rectified to provide a DC signal within a range suitable for use by motors or circuits having specified power requirements. The step down in voltage is accomplished by a transformer that steps the relatively high voltage provided by the electric utility to a lower voltage alternating current signal. The output of a transformer secondary is rectified to produce a pulsed DC signal which is filtered to provide a DC output.
The DC output from filtering circuitry is often used in conjunction with integrated circuit voltage regulators to provide well regulated voltages for use in digital circuit energization. Relatively inexpensive off the shelf voltage regulators for this purpose are known in the prior art. Such voltage regulators are designed to work over a range of d.c. voltages at their inputs.
Power supplies are rated for the amount of power (typically specified in watts) they can provide at their output. If the circuit load placed on the power supply calls for more energy than this rating, the power supply may be unable to service the component. As an example, a DC motor drawing too high a current through the motor windings can exceed the power supply output. Since the power supply should accommodate changes in load and energization signals, one goal of a power supply is the provision of a desired output voltage over a range of input and output conditions.
It is known in the prior art to couple power supply transformer windings in either parallel Or series to control outputs from a power supply. U.S. Pat. No. 3,984,750 to Pfeffer et al which issued Oct. 5, 1976 concerns a rectifier unit with phase windings which are switched from series to parallel configuration in response to an alternator frequency. By controllably switching the output from the phase windings, the manner in which battery charging occurs can be adjusted based upon engine speed. A preferred use of the circuit disclosed in Pfeffer et al is in charging a motor vehicle battery.
U.S. Pat. No. 4,339,704 to McSparran et al entitled "Series Parallel Transition For Power Supply" which issued Jul. 13, 1982 concerns a multi-winding, alternating current electric power supply. A load is coupled to the power supply by a power rectifier assembly having two legs coupled across the load. The legs are alternately coupled either in parallel or in series by a switch which controllably changes from the parallel to the series coupling. The switch between parallel and series coupling is controlled to maintain the torque output from a diesel powered locomotive motor.
U.S. Pat. No. 3,938,031 to Blackmond which issued Feb. 10, 1976 concerns an adjustable voltage power supply where the output from the supply is an alternating current signal. This signal is used to energize a resistance such as a heating element. The '031 patent includes a switching device that is controlled in a higher voltage range to produce a series winding connection at a predetermined point during each half cycle of a voltage output. The claim is made that the '031 apparatus operates with a lower maximum power output which can be maintained over a desired voltage range.